Mesopelagic fishes in the southern Benguela ecosystem: acoustic target strength and biomass

Abstract

Mesopelagic fishes such as Hector’s lanternfish Lampanyctodes hectoris and the Walvis pearlside Maurolicus walvisensis are ecologically important and potentially valuable marine resources in the southern Benguela ecosystem. Reliable biomass estimates are, however, required as inputs into ecosystem and stock assessment models to ensure their potential future sustainable exploitation. This study presents species-specific target strength–fish length relationships derived from in situ data, and acoustic biomass estimates from fisheryindependent surveys conducted between 2006 and 2022. The estimated combined biomass of these two species averaged ~1 million tonnes, but the annual estimates were highly variable, with M. walvisensis generally dominant. Despite the region’s high productivity, the density estimates were lower than those reported elsewhere. Key uncertainties included limited offshore spatial coverage and potential depth-related resonance at 38 kHz; however, diel and multifrequency analyses indicated minimal resonance-related bias within the surveyed depth range.